To maintain the safety of a process plant, a process plant may include an integrated or separately-controlled emergency shutdown system. An emergency shutdown system is intended to help contain a hazardous condition that may occur in or around a process plant. For example, some process plant systems, such as oil refineries or natural gas processing plants may require a quick shutdown if a fire were to break out in or around the process plant. An emergency shut down system may shut down the operation of the process plant by cutting off further supply of materials to the process plant. The system may take over the operation of the process plant when it detects errors or dangerous conditions existing in an area of the process plant and may shut down a main line valve that controls the supply of input materials to the process plant. The emergency shutdown system may employ a valve controller that shuts a supply valve of the process plant to prevent further flow of input material through the valve, which may prevent further flow of the material to the remainder of the process plant.
Some emergency shutdown systems may deteriorate over time and, in particular, may deteriorate to a state where the emergency shutdown system fails to operate properly. For example, in a pneumatic actuator used to close a highly-pressurized valve, dirt or other foreign materials may accumulate in the actuator or in the valve such that the actuator or the valve may fail or become stuck. Of course, if the actuator or valve that fails or becomes stuck (e.g., due to corrosion, accumulation foreign materials, etc.) the actuator or valve may not operate properly in response to a control signal.
In view of the potential for the emergency shutdown systems or components to fail at a crucial time if they are not properly tested and maintained, it is often desirable to test systems and components (e.g., valves, controllers, actuators) making up an emergency shutdown system, to ensure that the components will operate properly in an emergency. One method of testing the operation and/or response of a device to a shut down command is to issue a shut down command and determine whether the device responds to the command. In the case of an emergency shut down valve, this may include issuing a command to fully close the valve and determining whether the valve closes as expected. However, a test that involves completely closing a valve may be costly and may cause a large disturbance in the operation of the process plant.
An alternative to the complete shutdown test is to attach a bypass valve as a safety valve so that the safety valve can be fully shut down by what is referred to as a “full stroke” test without shutting down the process plant. However, there may be economic and operational problems associated with the bypass valve technique. Therefore, conventional diagnostics techniques have been developed that close a valve to be tested to a certain percentage toward a full closure of the valve. These tests are referred to as partial stroke (closure) tests.
However, problems may exist with conventional partial stroke tests. In particular, after the initiation of a conventional partial stroke test, it may take some time before a valve controller physically moves the valve off of the valve's travel stop. Some time may be required to transfer a user-initiated partial stroke test signal into a visible indication that the test is in progress. Because conventional emergency shutdown systems may not provide an immediate indication to a user that the user has initiated a partial stroke test, a user may not think the test has been initiated. Therefore, the user may try to initiate the test again by hitting a start test button for a second time in an effort to start the test while not having yet received an indication that the partial stroke test actually has been initiated. An additional problem may occur with conventional test systems if the user repeatedly hits the start test button. Because the user may not have an indication that the test system initiated his test, the user may hit a start test button again, but in actuality, the user may be turning on and off the test without the user being aware of his unintended actions.